Variable reactance



Feb. 15, 1944. w VAN ROBERTS I 2,341,655

VARIABLE REACTANCE Filed Aug. 25, 1942 CHOKE OSCILLATOR 0* Z TANK. E F L T T INVENTOR.

Ilium? MN 5. Rose-firs.

ATTORNEY Z is an inductance.

Patented Feb. 15,

2,341,655 VARIABLE announce Walter van B. Roberts, Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application August 25, 1942', Serial No. 456,018 2 Claims. (01. 179-1715) In this application I have disclosed a new and improved reactance tube arrangement wherein a tube'is so arranged and connected that a substantially constant conductance component together with a variable susceptance component is produced across two terminals at which alternating voltage appears. The susceptance-component may be varied by varying the tube plate resistance, and in my improved circuit means is provided for producing said variations of susceptance without causing at the same time changes in the conductance component.

Such an arrangement is of particular value in oscillator frequency control systems because by the use of the same, frequency control of the oscillations is accomplished without change in the amplitude thereof.

In describing my invention in detail reference will be, made to the attached drawing wherein Figure 1 is a basic tube circuit adapted to be arranged and operated to provide a variable susceptance component without substantial varia-' of the tube circuit of Figure 1 when Z is a pure reactance.

Figure 3 is an illustrative practical embodiment of my novel controlled reactance arrangement including means for supplying modulation voltage and adapted to be connectedto a wave source to control the wave length of the same without changing substantially the amplitude thereof. In Figure 1, Z is assumed to be a pure reactance in which casethe efiective admittance to the voltage E is shown in Figure 2 by the circle diagram, the various points on the circle corresponding to various values of plate resistance R. of the tube. This diagram is for the case where However, Z could be 9. capacity with the result that the circle would be chosen preferably by choosing Z to satisfy the condition R=|Z|( +2) I That I have provided a tube circuit wherein a constant conductance and variable susceptance is developed is proven mathematically hereinafter.

In Figure 1 assume Z is a pure reactance.

Rp is the tube plate impedance, and i is the plate current, and a the amplification constant of the tube.

From elementary considerations,

a: [E-e- (e) 1+ R,+z but, e=i Z is the voltage across Z inFi l-l e 2.

above the horizontal axis of the diagram. Fur- Y thermore, 2 may be complex and still a circle diagram will result from varying R. So long as a circle results which has a vertical portion, the arrangement has the property that a small variation in R will vary the susceptance of the system without substantially changing the conductance. Hence, if the operating point is chosen to fall on the vertical part of the circle diagram,

the' arrangement can be used-as a reactance tube to frequency modulate an oscillator, for example, without introducing appreciable amplitude modulation. Theproper operating point is Figure 3 shows a practical embodiment of the invention where an inductance L is used in the cathode circuit and a capacityC is put in shunt to the whole system. This capacity is chosen to wipe out all susceptance from the system in the absence of control voltage at AF. Thus, if the set-up is used for-FM, the mean frequency of the oscillator is not affected by the connection of the reactance tube system. a

The required value of L to provide constant conductance is given by +2 where f is the mean frequency, and Rp is the value of plate resistance corresponding to a desired bias on the tube. C is then chosen to sat- It is also possible to employhapacitive reactance in Z and to shunt the arrangement with an inductance in place of the element C of Figure 3.

However, the first discussed arrangement is pre-' ferred since the inductance in the cathode lead provides a direct current path'for space current,

posing modulating voltages on said bias whereby an effective susceptance is developed between anda controlgrimapairotterminalsbehem' whicha variable susceptancedsto. be produced, a low impedance connection anode and one terminal, a connection including reactance between the cathode and other terminaL a direct current source connected with the anode and cathode, means for supplying an. operating.

bias between the arid and cathode, meansjor superposing modulating voltage on said bias whereby susceptance is developed between said terminals, said reactance being chosen to satisfythe equation R,=|Z|( +2) (where Rp is the-tube said terminals, said reactance being chosen to satisfy the equation R,= |Z|(u+2) (whereR is the tube anode resistance, Z is the reactance oi the cathode lead and a lathe amplification constant of said tube) whereby said modulating voltage varies said eilective susceptance without substantially varying the effective conductance 20 between said terminals.

2. In a system of the nature described, a tube having electrodes including an anode. a cathode,

anode resistance, Z is the reactance of the oathode lead and a is the amplification constant of said tube), a susceptance of sign opposite to that in the said cathode lead connected across said terminals and of a size such as to make the etiective susceptance between said terminals are in the absence of odulating voltages whereby alternating current modulating voltages cause solely alternating values of susceptance between said terminals.

WALTER VAN B. ROBERTS. 

